Multi-array parallel optical links

ABSTRACT

An optical interconnect may provide for optical communications between two IC chips. The optical interconnect may include an array of optoelectronic elements, for example microLEDs and photodetectors, with the array including a plurality of sub-arrays. A fiber bundle of optical fibers may couple the optoelectronic elements, and the fiber bundle may include a plurality of sub-bundles, with for example one sub-bundle for coupling pairs of sub-arrays. Fibers of each sub-bundle may be accurately positioned with respect to one another.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims the benefit of U.S. Provisional PatentApplication No. 63/186,706, filed on May 10, 2021, the disclosure ofwhich is incorporated by reference herein.

FIELD OF THE INVENTIONS

The present invention relates generally to semiconductor chip-to-chipcommunications and more particularly to optical interconnects betweensemiconductor chips in different semiconductor packages.

BACKGROUND OF THE INVENTION

Computing and networking performance requirements are seeminglyever-increasing. Prominent applications driving these requirementsinclude data center servers, high-performance computing clusters,artificial neural networks, and network switches.

For decades, dramatic integrated circuit (IC) performance and costimprovements were driven by shrinking transistor dimensions combinedwith increasing die sizes, summarized in the famous Moore's Law.Transistor counts in the billions have allowed consolidation onto asingle system-on-a-chip (SoC) of functionality that was previouslyfragmented across multiple ICs.

However, Moore's Law appears to be reaching its limits as shrinkingfeature sizes below 10 nm results in decreasing marginal performancebenefits with decreased yields and increased per-transistor costs.Beyond these limitations, a single IC can only contain so muchfunctionality, and that functionality is constrained because the IC'sprocess cannot be simultaneously optimized for different functionality,e.g., logic, DRAM, and I/O. Increasingly, improving system performanceis dependent on implementing very high bandwidth interconnects betweenmultiple ICs.

Unfortunately, compared to the on-chip connections, today's chip-to-chipconnections are typically much less dense and require far more power(for example normalized as energy per bit). These inter-IC connectionsare currently significantly limiting system performance. Specifically,the power, density, latency, and distance limitations of interconnectsare far from what is desired.

New interconnect technologies that provide significant improvements inmultiple performance aspects are highly desirable. It is well-known thatoptical interconnects may have fundamental advantages over electricalinterconnects, even for relatively short interconnects of <<1 meter.Unfortunately, implementation of optical interconnects for inter-ICconnections may face a host of problems. Included in these problems isthat of coupling light from one IC to another IC. Electricalinterconnect technology for inter-IC communications at a substrate orcircuit board level may be relatively well-developed. The same may notbe as true for optical interconnect technology for inter-ICcommunications, particularly for high-throughput applications thatpreferably do not negatively impact existing modes of electricalinterconnections.

BRIEF SUMMARY OF THE INVENTION

Some embodiments of a parallel optical interconnect provide: a firstoptical transceiver array comprising a plurality of optical transmittersand receivers, a first optical coupling assembly for optically couplingthe first optical transceiver array to a first end of a fiber bundle,the fiber bundle comprising a plurality of fiber cores, a second opticaltransceiver array, and a second optical coupling assembly for opticallycoupling that second array to a second end of the fiber bundle. In someembodiments the first optical transceiver array comprises a plurality oftransceiver sub-arrays. In some embodiments each transceiver sub-arraycomprises a plurality of light emitters and a plurality ofphotodetectors. In some embodiments the light emitters are microLEDs. Insome embodiments the fiber bundle comprises a plurality of fibersub-bundles, each fiber sub-bundle comprising a plurality of fibercores. In some embodiments adjacent fiber cores in a fiber sub-bundleare arranged closer in space than adjacent fiber cores in differentfiber sub-bundles. In some embodiments there is a one-to-onecorrespondence between transceiver sub-arrays of a transceiver array andfiber sub-bundles of a fiber bundle. In some embodiments emitters of oneoptical transceiver array are paired with photodetectors of anotheroptical transceiver array. In some embodiments there is a one-to-manycorrespondence between fiber cores and emitter/photodetector pairs. Insome embodiments there is a one-to-one correspondence between fibercores and emitter/photodetector pairs.

In some embodiments of a parallel optical interconnect, each opticaltransceiver array comprises one or more transceiver sub-arrays, whereeach sub-array comprises a plurality of optical transmitter andreceivers. In some embodiments, the fiber bundle comprises an array ofsub-bundles, where each sub-bundle comprises a plurality of fiber cores.In some embodiments, the shape and position of the fiber-sub-bundles ismatched to that of the transceiver sub-arrays such that each transceiversub-array is coupled to one end of a corresponding fiber sub-bundle.

In some embodiments, the transceiver sub-arrays and fiber sub-bundlesmay be arranged on a two-dimensional grid. In some embodiments the gridmay be a regular grid on a square grid, hexagonal close-packed (HCP,equivalent to an equilateral triangle grid) grid, some other regularpolygonal grid, or may be an irregular grid. In some embodiments theremay be a gap between sub-arrays. In some embodiments the sub-arrays maybe contiguous.

In some embodiments, each transceiver sub-array and ends of each fibersub-bundle may define an approximately square or rectangular shape. Insome embodiments, each sub-array and ends of each sub-bundle may definean approximately hexagonal shape. In some embodiments, each sub-arrayand ends of each sub-bundle may define an approximately triangularshape. In some embodiments, each sub-array may define a shape that isrotationally asymmetric. Such rotationally asymmetric sub-arrays maylend themselves more easily to being accurately oriented rotationally.

In some embodiments, each sub-array may comprise a mix of one or moretransmitters and one or more receivers. In some embodiments, eachsub-array may comprise just transmitters or just receivers. In someembodiments, each sub-array may comprise an equal number of transmittersand receivers. In some embodiments, each sub-array may comprise anunequal number of transmitters and receivers. In some such embodiments anumber of transmitters may be a multiple of a number of receivers. Insome such embodiments, each sub-array may comprise a number of emittersthat is a multiple of a number of detectors.

Some aspects of the invention provide an optical interconnect,comprising: a first IC chip having a first plurality of sub-arrays ofoptoelectronic elements; a second IC chip having a second plurality ofsub-arrays of optoelectronic elements; and a fiber bundle including aplurality of fiber sub-bundles, each of the fiber sub-bundles comprisedof a plurality of fibers, each of the plurality of fibers including acore concentrically surrounded by cladding, with optoelectronic elementsof different ones of the first plurality of sub-arrays of optoelectronicelements optically coupled to optoelectronic elements of different onesof the second plurality of sub-arrays by fiber elements of differentones of the fiber sub-bundles; with fiber elements of each fibersub-bundle arranged on a grid.

In some aspects, each fiber sub-bundle comprises in inner regioncontaining the fiber elements and an outer region, the outer regionhaving dimensions that are independent of variations in size andposition of the fiber elements of the fiber sub-bundle. In some aspectsthe grid is a hexagonal close packed grid. In some aspects the grid is asquare grid. In some aspects the optoelectronic elements comprisemicroLEDs and/or photodetectors. In some aspects the sub-bundles areaccurately positioned relative to each other only near the fiber elementends. In some aspects the fiber bundle includes at least one fiducialfeature.

Some aspects further comprise a first substrate with a plurality ofapertures and a second substrate with a plurality of apertures, each ofthe plurality of apertures of the first substrate holding first ends ofcorresponding ones of the fiber sub-bundles and each of the plurality ofapertures of the second substrate holding second ends of thecorresponding one of the fiber sub-bundles. In some such aspects theapertures are rotationally asymmetric.

Some aspects further comprise filler material between fiber elements ofeach fiber sub-bundle.

In some aspects ends of the fiber bundles are circumferentially encasedby an outer jacket. In some such aspects the outer jacket includes aflat edge, allowing for determination of rotational orientation of thefiber bundle.

These and other aspects of the invention are more fully comprehendedupon review of this disclosure.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A shows a parallel optical interconnect using a fiber bundle, inaccordance with aspects of the invention.

FIGS. 1B and 1C show top views of embodiments of a transceiver IC, inaccordance with aspects of the invention.

FIGS. 2A and 2B show example ends of fiber bundles, in accordance withaspects of the invention.

FIG. 3 shows an example matching of light emitter and/or detectorsub-arrays with sub-bundles of a fiber bundle, in accordance withaspects of the invention.

FIG. 4A and FIG. 4B are block diagrams of a transmitter and of areceiver, respectively, in accordance with aspects of the invention.

FIGS. 4C and 4D are top views of transceiver ICs, or portions oftransceiver ICs, showing example layouts of optoelectronic elements, inaccordance with aspects of the invention.

FIGS. 5A-D show emitter and photodetector structures on or in an IC, inaccordance with aspects of the invention.

FIGS. 6A-D illustrate cross-sections of single core fibers and fibersub-bundles.

FIGS. 7A and 7B illustrate a cross-section of a sub-bundle in asquare-packed grid and a cross-section of a sub-bundle in a HCP grid,respectively, with filler material between fiber elements, in accordancewith aspects of the invention.

FIG. 8 illustrates a fiber bundle comprised of a plurality ofsub-bundles 811 with attachment structures about opposing ends of thefibers of the sub-bundles, in accordance with aspects of the invention.

FIGS. 9A-C illustrate fiber sub-bundles with defined outer dimensions,in accordance with aspects of the invention.

FIGS. 10A-C illustrate example attachment structures for a fiber bundle,in accordance with aspects of the invention.

DETAILED DESCRIPTION

FIG. 1A shows a parallel optical interconnect using a fiber bundle. Theparallel optical interconnect may connect two semiconductor packages.The semiconductor packages may be packages for a single semiconductorchip, or, in many embodiments, may be packages of multi-chip modules.The packages each include a transceiver array 111 a,b. Each transceiverarray may include light emitters and/or photodetectors on a top side (asillustrated in FIG. 1A) of an integrated circuit chip or die (which maybe referred to as an IC). In some embodiments the photodetectors may beintegrated into the IC. The IC may be a separate transceiver IC in thepackage, or the IC may be an IC with other functions, for exampleprocessing or memory functions.

The light emitters and photodetectors are positioned for, respectively,provision or reception of light in a direction towards or from what maybe considered a top of their respective packages. Coupling optics 113a,b within the package, however, direct the light so that the lighttransfers instead through a connection in a side wall of the packages.The coupling optics may include, for example, a turning mirror 115 a,bto change a direction of the light from a vertical direction to ahorizontal direction. Transfer of the light through the connection inthe side wall of the packages may be beneficial in avoiding interferencewith placement of heat transfer elements that may be present on a topside of the packages, for example.

A fiber bundle 117 couples to the coupling optics of each of thesemiconductor packages. The fiber bundle includes a plurality of fibercores for transfer of light between opposing ends of the fiber bundle,and thence to and from the coupling optics. The fiber bundle may includea plurality of sub-bundles, each of which includes a plurality of fibercores. In some embodiments light emitters and/or photodetectors in eachpackage are arranged in an array, with each array including a pluralityof sub-arrays of light emitters and/or photodetectors. In someembodiments each sub-array is associated with a corresponding fibersub-bundle, with the fiber sub-bundle carrying light for that sub-array.

FIGS. 1B and 1C show top views of embodiments of a transceiver IC 121a,b. The transceiver ICs may be a transceiver IC as discussed withrespect to FIG. 1A. The transceiver ICs include a plurality ofsub-arrays 123 a,b of light emitters and/or photodetectors on a top ofthe transceiver IC. In FIG. 1B, the sub-arrays define a square shape foran array of light emitters and/or photodetectors. In some embodimentsthe sub-arrays (and array) may instead define a rectangular shape. InFIG. 1C, the sub-arrays define a hexagonal shape.

FIGS. 2A and 2B show example ends of fiber bundles. In both FIGS. 2A and2B, a plurality of fiber sub-bundles 211 a,b are surrounded by fillermaterial 213. The sub-bundles and filler material are circumferentiallyencased by an outer jacket 215 a,b. The ends of the sub-bundles of FIGS.2A and 2B define shapes corresponding to the shapes of the sub-arrays ofFIGS. 1B and 1C, respectively. In FIG. 2A, ends of the sub-bundlesdefine a square shape, with the square shapes of the sub-bundles in turndefining a square shape for cores of the fiber bundle. In FIG. 2B, endsof the sub-bundles define hexagonal shapes. The outer jacket may definea circular shape in some embodiments, and as illustrated in FIG. 2A. Insome embodiments the outer jacket may include a flat edge 219, forexample as illustrated in FIG. 2B. The use of the flat edge may bebeneficial in, for example, allowing for determination of rotationalorientation of the fiber bundles, which may provide for increasedability to align ends of the sub-bundles with corresponding emitterand/or photodetector sub-arrays.

In some embodiments, the fiber bundle may comprise fiducial featuresthat allow the rotational and/or translational orientation of the fiberbundle to be uniquely determined. In some embodiments, the outer profileof the fiber bundle may include one or more fiducial markers. In someembodiments, ends of the fiber bundle may include one or more fiducialmarkers 217, for example as shown in FIG. 2A. The fiducial markers maybe, in some embodiments, differently colored elements, that enablevisual determination of the unique rotational and/or translationalorientation of the fiber bundle.

FIG. 3 shows an example matching of light emitter and/or detectorsub-arrays with sub-bundles of a fiber bundle. In FIG. 3 , a pluralityof square-shaped sub-arrays 123 a of a transceiver IC are eachassociated with a corresponding fiber sub-bundle 211 c of a fiberbundle, for example as indicated by arrow 311 marking an associationbetween one of emitter and/or detector sub-arrays and one of the fibersub-bundles. As in FIG. 2A, the fiber bundle includes a plurality offiber sub-bundles, arranged in a square array, with the sub-bundleslongitudinally surrounded by filler material 213. An outer jacket 215circumferentially bounds the sub-bundles and filler material. In someembodiments a single fiber core of the sub-bundle receives light from asingle emitter of the sub-array or passes light to a single detector ofthe sub-array. In embodiments such as the embodiment of FIG. 1A, thesingle fiber core may have a first end for doing so for an emitter ordetector of the transceiver array of a first IC and a second end fordoing so for an emitter or detector of the transceiver array of a secondIC.

FIG. 4A is a block diagram of a transmitter. In some embodiments eachtransmitter comprises an optical emitter 413 and transmitter circuitry411. The transmitter circuitry receives an input electrical signal, forexample a data signal or data signals. In some embodiments thetransmitter circuitry drives the emitter to encode the data signal ordata signals into light signals emitted by the emitter. In someembodiments, the transmitter emitter elements are microLEDs. In someembodiments the microLEDs are comprised, for instance, of GaN, GaAs, orInP. In some embodiments each transmitter circuit may comprise anemitter driver and an equalizer. In some embodiments the equalizer maycompensate the frequency response of the emitter being driven, forinstance compensating for the high-frequency roll-off of the emitterfrequency response.

FIG. 4B is a block diagram of a receiver. In some embodiments eachreceiver comprises a photodetector (PD) 421 and receiver circuitry 423.The PD converts light signals into electrical signals, which areprocessed by the receiver circuitry, for example to amplify and in someembodiments equalize the electrical signals. In some embodiments, the PDmay be comprised, for instance, of Si, Ge, GaAs, SiGe, or InP. Each PDgenerates a photocurrent in response to light received from atransmitter, so the PD material preferably should be chosen to besensitive in the wavelength range emitted by the transmitters. In someembodiments the transmitters emit light in a blue wavelength range. Insome embodiments the transmitters emit light in a wavelength range of250 nm to 700 nm. In some embodiments the transmitters emit light in awavelength range of 375 nm to 600 nm. In some embodiments thetransmitters emit light in a wavelength range of 400 to 500 nm. In someembodiments, the PDs may be silicon photodetectors, for instance p-i-nphotodetectors or avalanche photodiodes. In some embodiments eachreceiver circuit may comprise a transimpedance amplifier (TIA), alimiting amplifier, a buffer amplifier to drive output loads, and anequalizer, for example to compensate the link frequency response, forinstance to increase the bit rate the link can support.

The optical emitters and photodetectors may be referred to collectivelyas “optoelectronic elements” herein. FIGS. 4C and 4D are top views oftransceiver ICs, or portions of transceiver ICs, showing example layoutsof optoelectronic elements. In FIG. 4C, a transceiver IC 451, or aportion of a transceiver IC, includes emitters 413 and PDs 421 arrangedin a rectangular pattern, with two rows of PDs followed by two rows ofemitters shown in FIG. 4C. In FIG. 4D, transceiver IC 451, or a portionof a transceiver IC, includes emitters 413 and PDs 421 arranged in ahexagonal close packed (HCP) grid, with rows of PDs followed by rows ofemitters. In some embodiments, accordingly, the optoelectronic elementsin a sub-array may be arranged in a regular pattern, e.g., a square,rectangular, or HCP. In some embodiments, and as shown in FIGS. 4C and4C, emitter widths are smaller than PD widths. Smaller emitter widthsmay allow for higher coupling efficiency to the fiber bundle usingper-emitter collection optics that preserve source etendue by allowingthe beam to expand while reducing its angular spread. Using PDs withlarge widths may enable relaxed (i.e., larger) translational alignmenttolerances for the PDs relative to the fiber bundle. In someembodiments, some subset of the optoelectronic elements may be on aregular grid with some omitted elements. For example, FIG. 4D shows PDand emitter elements on a HCP grid with some omitted elements betweenthe PDs and emitters.

FIGS. 5A-D show emitter and photodetector structures on or in an IC, inaccordance with aspects of the invention. In some embodiments, thetransceiver array may comprise a substrate; in some embodiments, thesubstrate comprises an IC, where the IC comprises one or more instancesof transmitter and a receiver circuitry. In some embodiments of atransceiver array, each emitter may be attached to the IC. FIG. 5A showsa cross-sectional view of an example emitter. The emitter may be, forinstance, a microLED 513 solder-bonded 517 to an IC pad 519, of a topmetal layer 515 of an IC 511. The emitter may be driven by a transmittercircuit, for example the transmitter circuit of FIG. 4A. In someembodiments, the emitters emit light preferentially in a directionnormal to the IC surface. In some embodiments, one or more opticalelements such as a lens and/or reflector structure are attached to orcoupled to the IC surface, or in or to a structure attached to orcoupled to the IC surface, in the vicinity of each emitter. In someembodiments the optical element(s) and help to collect light from eachemitter such that angular spread of optical distribution of light fromeach emitter is reduced.

FIG. 5A illustrates a cross-section of a PD monolithically integrated inan IC. The PD 521 may be in a top of the IC 511. An IC top metal layer515 may provide electrical connections to the PD, with in someembodiments the top metal layer not blocking or substantially notblocking light from reaching the PD. An output of the PD may beconnected to a TIA input (not shown in FIG. 5A), which may be in the IC.Such an embodiment may be especially useful for materials in whichhigh-performance, high-density, low-cost electronics are available suchas silicon. In other embodiments, for example as illustrated in thecross-section of FIG. 5C, each receiver in the IC 511 comprises a PD 531that is bonded to the IC, for instance using solder 533 coupled to thetop metal layer 515 of the IC. One of the PD pads may be electricallyconnected to an input of a TIA in the IC. In some embodiments, forexample as illustrated in a top view of FIG. 5D, the PDs in an IC 541may utilize a lateral structure with interdigitated fingers extendingfrom an n-contact 545 and a p-contact. In some embodiments, opticalelements such as a lens and/or reflector structure are attached to orcoupled to the IC surface in the vicinity of each PD and help to collectlight from an area larger than that of the PD and cause that light to beincident on the PD's surface.

In some embodiments, the fiber bundle comprises a plurality ofsingle-core fiber elements (FEs) arranged in a bundle. FIG. 6Aillustrates a cross-section of a single core fiber. In some embodimentseach single core fiber comprises a circular core 611 concentricallysurrounded by a cladding 613, where the refractive index of the claddingis less than that of the core such that light coupled into the core isguided down the core via total internal reflection for light rays thatare incident on the core-cladding interface at an angle (relative to theinterface normal) that is greater than a “critical” angle, beyond whichlight rays experience total internal reflection (TIR). The fiber coreand cladding may be made from a glass, or may be made from some othermaterial such as a polymer.

In some embodiments, each FE additionally comprises a jacket 615 thatconcentrically surrounds the cladding, as illustrated in thecross-section of FIG. 6B. This jacket may be made from a variety ofmaterials, for instance glass or a polymer.

In some embodiments, a fiber bundle comprises one or more “sub-bundles.”In some embodiments each sub-bundle is a fiber bundle as describedpreviously. In some embodiments, the FEs within each bundle orsub-bundle are arranged on a grid that is approximately square, forexample as illustrated in FIG. 6C. In some embodiments, the FEs within abundle or sub-bundle are arranged on a HCP grid, for example asillustrated in FIG. 6D.

In some embodiments, the bundle (or sub-bundle) of FEs is fabricated ina manner such that the cladding of adjacent fibers is slightly meltedtogether to adhere the FEs to each other. In some embodiments,additional filler material occupies some or all of the space between theFEs. FIGS. 7A and 7B illustrate a cross-section of a sub-bundle in asquare-packed grid and a cross-section of a sub-bundle in a HCP grid,respectively, with filler material 715 between fiber elements. The fiberelements generally include a core concentrically surrounded by cladding713. In some embodiments, the additional filler material between the FEsis approximately circular in cross-section. This filler material may bea glass, a polymer, or some other material.

In some embodiments, the diameter of each FE core is in the range of 1um to 10 um. In some embodiments, the diameter of each FE core is in therange of 10 um to 100 um. In some embodiments, the diameter of each FEcore is greater than 100 um. In some embodiments, the ratio of thecladding diameter to the core diameter for each FE is in the range of1.0 to 1.2. In some embodiments, the ratio of the cladding diameter tothe core diameter for each FE is >1.2.

In some embodiments, the ratio of the jacket diameter to the claddingdiameter for each FE is in the range of 1.0 to 1.2. In some embodiments,the ratio of the jacket diameter to the cladding diameter for each FE is>1.2.

In some embodiments of a fiber bundle comprised of sub-bundles, thesub-bundles are attached to each other only along a limited length nearthe fiber end faces, and are not attached to each other along otherparts of the length of the fiber. FIG. 8 illustrates a fiber bundlecomprised of a plurality of sub-bundles 811. In FIG. 8 , attachmentstructures 813 a,b are about opposing ends of the fibers of thesub-bundles.

In some embodiments of a parallel optical interconnect, the light fromeach transmitter is coupled into multiple FEs; at the other end of thefiber bundle, the light from the multiple FEs into which a transmitter'slight was coupled is coupled to a single PD.

In some embodiments of a parallel optical interconnect, the light fromeach transmitter is coupled to a single FE; at the other end of thefiber bundle, the light from the FE is coupled to a single PD. In suchembodiments, each FE preferably is accurately positioned relative to thetransceiver element (emitter or PD) to which it is optically coupled. Inturn, each sub-bundle comprising an overall fiber bundle preferably isaccurately positioned with respect to transceiver elements orsub-arrays.

The fiber bundles discussed above may be formed by stacking fibers invarious ways. Variances in the diameter and position of each FE maycause significant variance in location of a given core in a fiber bundlerelative to its “ideal” position on a grid due to accumulated errorsacross the bundle. As the number of fibers within a bundle is increased,the accumulated position error for each core grows. If there is amaximum core positioning error requirement, this may limit the maximumuseful number of FEs in a bundle.

These FE variances can also cause the cross-sectional size of a fibersub-bundle to vary from its designed size. Similar to the problem ofaccumulated FE size/position errors within each sub-bundle, creating afiber bundle by stacking multiple fiber sub-bundles can cause anaccumulated error in the position of each sub-bundle relative to itsideal position. This error is added to the FE position error within asub-bundle and can quickly result in large positioning errors for theFEs within each bundle relative to an “ideal” grid.

For a fiber bundle comprised of multiple sub-bundles, the error inposition of each sub-bundle can be minimized by using a mechanism forpositioning each sub-bundle that does not depend on the positions ofother sub-bundles. In some embodiments of a fiber bundle comprised of anarray of sub-bundles, the sub-bundles may be separated by a fillermaterial, for example as discussed with respect to FIGS. 7A, B. In someembodiments, the width of the filler material may be adjusted to offsetany variance in cross-sectional size of each sub-bundle relative to thedesigned size of that sub-bundle such that each sub-bundle is accuratelypositioned relative to its “ideal” position.

In some embodiments, the sub-bundles are accurately positioned relativeto each other only over a limited length near the fiber end faces, forexample as discussed with respect to FIG. 8 , whle the rest of thelength of the sub-bundles are not tightly attached to each other suchthat their positions relative to each other may change as the fiber ismoved or manipulated.

In some embodiments, each sub-bundle comprises an inner region thatcontains the FEs and an outer region with accurate outer dimensions thatare independent of variations in size and position of the FEs comprisingthe sub-bundle. FIG. 9A shows an embodiment of a sub-bundle comprisingan inner region 911 comprising a plurality of FEs 913 and an outerregion 915 concentrically surrounding the inner region. The outer regionhas a circular cross-section of diameter D, where D is independent ofthe inner region details. FIG. 9B shows an embodiment of a sub-bundlethat is similar to FIG. 9A, with an inner region 923 comprised of aplurality of FEs 921, and an outer region 925 concentrically surroundingthe inner region. In FIG. 9B, however, the outer region has a hexagonalcross-section of width W that, like the outer region of FIG. 9A, isindependent of the details of the inner region. FIG. 9C shows a fiberbundle comprised of an outer region 937 concentrically surroundingsub-bundles 935 of the form of the sub-bundle of FIG. 9B, with accurateouter dimensions. Such a bundle has FE positions that are not impactedby accumulated errors due to FE positions in other sub-bundles. Byaccurately controlling the outer dimensions of each sub-bundle, fiberbundles with large numbers of FEs can be constructed where the error inthe position of each FE relative to an “ideal” grid can be held within adesired bound.

FIGS. 10A-C illustrate example attachment structures for a fiber bundle.FIGS. 10A and 10B show a face view and a cross-sectional side view,respectively, of a substrate 1011 providing an attachment structure. Insome embodiments of a fiber bundle, the end section of each fibersub-bundle 1015 is inserted into an aperture 1013 in the substrate,where each aperture traverses the substrate thickness. In someembodiments, the size and shape of each aperture may be just slightlylarger than that of the sub-bundle it holds such that the position ofthe sub-bundle is accurately determined by the aperture. If eachaperture within the substrate has high positioning accuracy, for examplesufficient to align the sub-bundle with an optoelectronic sub-array,this structure can be used to achieve accurate positioning of the endface of each fiber sub-bundle. In some embodiments, each aperture has acircular cross-section. In some embodiments, the aperture cross-sectionmay approximate the sub-bundle cross-section. In some embodiments theapertures and the sub-bundles may be rotationally asymmetric, forexample as shown in FIG. 10C. In FIG. 10C, an aperture 1033 in asubstrate 1031 is in the form of an irregular convex hexagon, and thesub-bundle cross-section 1037 approximates the irregular convex hexagon.The sub-bundle is comprised of FEs. If the sub-bundle cross-section isrotationally asymmetric and the aperture cross-section approximates thatof the sub-bundle, inserting the sub-bundle in the aperture may uniquelyrotationally orient the sub-bundle.

In some embodiments, the apertures in the substrate may be arranged on aregular grid, for example as illustrated in FIG. 10A. The grid is in thedesired arrangement (e.g., grid shape and spacing) of the sub-bundles.In some embodiments the substrate may be made from silicon, glass,ceramic, polymer, or some other material where apertures can befabricated with the desired high precision. In the case of a siliconsubstrate, in some embodiments the apertures may be formed through somestandard silicon micromachining process such as deep reactive ionetching (DRIE). In some embodiments each fiber sub-bundle may be fixedin place using an adhesive material applied between the sub-bundle andthe substrate.

Although the invention has been discussed with respect to variousembodiments, it should be recognized that the invention comprises thenovel and non-obvious claims supported by this disclosure.

1. An optical interconnect, comprising: a first IC chip having a firstarray of optoelectronic elements including a first sub-array ofmicroLEDs and a first sub-array of photodetectors; a second IC chiphaving a second array of optoelectronic elements including a secondsub-array of microLEDs and a second sub-array of photodetectors; and afiber bundle including a plurality of fiber sub-bundles, each of thefiber sub-bundles comprised a plurality of fibers, each of the pluralityof fibers including a core concentrically surrounded by cladding, withthe first sub-array of microLEDs being optically coupled to the secondsub-array of photodetectors and the second sub-array of microLEDs beingoptically coupled to the first sub-array of photodetectors by fiberelements of different ones of the fiber sub-bundles; wherein fiberelements of each fiber sub-bundle are arranged on a grid.
 2. The opticalinterconnect of claim 1, wherein each fiber sub-bundle comprises aninner region containing the fiber elements and an outer region, theouter region having outer boundary dimensions that are independent ofvariations in size and position of the fiber elements of the fibersub-bundle. 3.-4. (canceled)
 5. The optical interconnect of claim 1,further comprising filler material between fiber elements of each fibersub-bundle.
 6. The optical interconnect of claim 1, wherein the grid isa hexagonal close packed grid.
 7. The optical interconnect of claim 1,wherein the grid is a square grid.
 8. (canceled)
 9. The opticalinterconnect of claim 1, wherein the fiber sub-bundles are accuratelypositioned relative to each other only near ends of the fiber bundle.10. The optical interconnect of claim 1, wherein ends of the fiberbundle are circumferentially encased by an outer jacket.
 11. The opticalinterconnect of claim 10, wherein the outer jacket includes a flat edge,allowing for determination of rotational orientation of the fiberbundle.
 12. The optical interconnect of claim 1, wherein the fiberbundle includes at least one fiducial feature.
 13. An opticalinterconnect, comprising: a first IC chip having a first plurality ofsub-arrays of optoelectronic elements arranged in a first array shape; asecond IC chip having a second plurality of sub-arrays of optoelectronicelements arranged in a second array shape the same as the first arrayshape; and a fiber bundle including a plurality of fiber sub-bundles,each of the fiber sub-bundles comprise a plurality of fibers, each ofthe plurality of fibers including a core concentrically surrounded bycladding, with optoelectronic elements of different ones of the firstplurality of sub-arrays of optoelectronic elements optically coupled tooptoelectronic elements of different ones of the second plurality ofsub-arrays by fiber elements of different ones of the fiber sub-bundles;wherein the plurality of fiber sub-bundles are arranged on a grid havinga same shape as the first array shape and the second array shape. 14.The optical interconnect of claim 13, wherein the first plurality ofsub-arrays of optoelectronic elements comprises microLEDs and the secondplurality of sub-arrays of optoelectronic elements comprisesphotodetectors.
 15. The optical interconnect of claim 14, wherein thephotodetectors are monolithically integrated with the second IC chip.16. The optical interconnect of claim 14, wherein the microLEDs aresolder-bonded to a top metal layer of the first IC chip.
 17. The opticalinterconnect of claim 13, wherein the first array shape of the firstplurality of sub-arrays of optoelectronic elements is square-shaped. 18.The optical interconnect of claim 13, wherein the first array shape ofthe first plurality of sub-arrays of optoelectronic elements ishexagonal-shaped.
 19. The optical interconnect of claim 14, wherein themicroLEDs and the photodetectors are arranged in a rectangular pattern.20. The optical interconnect of claim 14, wherein the microLEDs and thephotodetectors are arranged in a hexagonal pattern.
 21. The opticalinterconnect of claim 13, wherein each of the fiber sub-bundles areseparated from each other by a filler material.
 22. The opticalinterconnect of claim 13, wherein each fiber sub-bundle comprises aninner region containing the fiber elements and an outer region, theouter region having outer boundary dimensions that are independent ofvariations in size and position of the fiber elements of the fibersub-bundle.